22 degrees is the maximum angle you can use without boundary separation in perfect conditions like a wind tunnel.
You can't have perfect conditions as you are not coming out to the edge of the van but instead leaving the stock rear trims that looks to have an angle greater than 22 degrees to start. You will have boundary separation at the rear of your van and your tail is just keeping the vortex from wrapping around as far.
Any particular reason you aren't going all the way to the edge of your van with your tail?
It looks like you are basically trying to make a trailer tail like those used on tractor trailers. You will find lots of info on them here:
https://nacfe.org/wp-content/uploads...Report2020.pdf
